In general, numerical control units, etc. for automatic machine control or sequential control units or other control units display the input and output state and internal state for diagnosis of components of the units, debugging of control programs, and the like.
In the above-mentioned method of display of conventional control units, the on or off state of output signals are indicated by lights or "1" or "0" are arranged on a CRT display for displaying the input and output states and internal state. Therefore, they only display the current input and output states. To resolve problems relating to timing, however, the operator needs to know the above states over a certain time interval. With the conventional method, where only the current states are displayed, it is difficult to detect by sight the length of on or off times of signals, time gaps between signals, etc. Therefore, conventional status display methods have the disadvantage that they are useless for resolving problems relating to timing.